1. The Field of the Invention
This invention generally relates to pyrolyzable polymers useful for producing articles comprising aluminum nitride. This invention particularly appertains to aluminum-nitrogen polymers prepared from the reaction of an organic nitrile and a trialkylaluminum reagent, and to aluminum nitride ceramics made from such polymers and from precursors therefor.
2. The State of the Art
Aluminum nitride exhibits a unique combination of physical properties, for example, a density of 3.26 g/cm.sup.3, a Young's modulus of 280 GPa, a flexural strength of 400 MPa, and a Knoop hardness of 1200 kg/mm.sup.2. Aluminum nitride (AlN) is also a refractory material that melts at approximately 2400.degree. C. As such, AlN is very stable in the presence of molten metals and can be used to fabricate various refractory articles for contacting molten metal, such as crucibles, gates, and nozzles.
Aluminum nitride is also an electrical insulator with a bandgap of 6.2 electron volts and has a coefficient of thermal expansion approximating that of silicon. Additionally, AlN has a thermal conductivity nearly ten times greater than alumina, a conventional substrate material used in electronic (e.g., microelectronic, microwave) packages. In high power microelectronic applications, significant amounts of heat are generated by the integrated circuit (IC), which typically comprises a silicon-based body. Removal of the heat generated, and resistance of mechanical stresses due to differing thermal expansion coefficients between the IC and the substrate to which it is bonded, are significant problems. Accordingly, the chemical and physical properties of AlN make it an attractive alternative to the conventional alumina and beryllia substrate materials. Although the thermal conductivity of beryllia is similar to that of aluminum nitride, beryllia presents practical problems because of its severe toxicity.
One drawback to using ceramics is the difficulty of fabricating high tolerance parts for both electronic and structural applications. Machining of ceramics is expensive and generates hazardous fines, especially when the ceramic includes reinforcing fillers or fibers such as silicon carbide whiskers. The art has resorted to compounding ceramic powder with a polymeric carrier to facilitate forming by injection molding, extruding, or casting (e.g., tape casting). With knowledge of the volumetric changes upon both curing of the polymer and sintering of the ceramic particles, high tolerance parts requiring minimal machining can be produced.
There is a continuing interest in polymer precursor materials that can be pyrolyzed directly to ceramic materials, and there is a current interest especially involving aluminum nitride ceramics. For example, aluminum-nitrogen polymers containing no alkyl substitution on the aluminum or nitrogen atoms are described in U.S. Pat. No. 4,747,607, in which thermolysis of a mixture of aluminum chloride and hexamethyldisilazane results in the polymeric --(Cl)Al-N(H)].sub.n. Pyrolysis of the polymer in ammonia or under vacuum yielded crystalline AlN. An infusible polymeric aluminum amidimide of the formula --(NH.sub.2)Al-N(H)].sub.n that was pyrolyzed to form AlN has been described by L. Maya, Adv. Ceram. Mat., 1986, 1, 150-153.
Polymers having the repeating unit --(R)Al-N(H)].sub.n are disclosed in U.S. Pat. No. 4,696,968 and in European Pat. Appln. No. 259,164; as described therein, fibers were melt spun from the thermoplastic precursor and pyrolyzed to form AlN fibers. L. V. Interrante et at., Inorg. Chem., 1989, 28, 252-257, and Mater. Res. Soc. Symp. Proc., 1986, 73, 359-366, both report the formation of volatile crystalline precursors than can be sublimed under vacuum. A two-step pyrolysis of these precursors in ammonia resulted first in an insoluble aluminum imide polymer of the form --(R)Al-N(H)].sub.n, and ultimately AlN containing less than 0.5% residual carbon and oxygen. U.S. Pat. No. 4,783,430 discloses the formation of --(CH.sub.3)Al-N(H)].sub.n which can be pyrolyzed under helium, argon, or vacuum to form hexagonal AlN.
Polymers having the repeating unit --(H)Al-N(R)].sub.n, as disclosed in U.S. Pat. No. 3,505,246, are formed by the reaction of the alane adduct H.sub.3 Al--N(C.sub.2 H.sub.5).sub.3 with a reagent such as acetonitrile. U.S. Pat. No. 4,687,657 discloses the preparation of a poly-N-alkyliminoalane that can be pyrolyzed in argon or under vacuum to form AlN.
Reacting an organic nitrile with trimethylaluminum produced organoaluminum imines that were not polymerized upon heating to a temperature of 280.degree. C. For example, J. R. Jennings et at., J. Chem. Soc., 1965, pp. 5083-5094, prepared an organoaluminum imine of the formula (CH.sub.3).sub.3 C(CH.sub.3).dbd.NAl(CH.sub.3).sub.2 by heating the adduct of tert-butylnitrile and trimethylaluminum to 150.degree. C. This imine remained unchanged after heating at 280.degree. C. for two hours under nitrogen. Although the authors describe the preparation of various aluminum-nitrogen "polymers" which comprise two organic groups on aluminum and a carbon-nitrogen double bond when decomposition below 280.degree. C. is observed, they acknowledge that only dimeric compounds were identified. J. E. Lloyd et at., J. Chem. Soc., 1965, pp. 2662-2668, describe reacting organoaluminum compounds with benzonitrile at temperatures of 190.degree.-200.degree. C. to yield the dimeric adduct [C.sub.6 H.sub.5 C(H).dbd.N--Al(C.sub.2 H.sub.5).sub.2 ].sub.2.
European Pat. Appln. No. 331,448 discloses that AlN can be vapor deposited onto a substrate by heating the substrate in the presence of the vapor of an aluminum-nitrogen compound having the formula CH.sub.3 (R.sup.1)Al-N(R.sup.2)(C.sub.3 H.sub.7), wherein R.sup.1 is alkyl and R.sup.2 is hydrogen, alkyl, or aryl. It is described by those applicants that a polymer of this compound may have been made, but neither the structure of the polymer nor any of its properties are disclosed.
Reacting an organic nitrile with diisobutylaluminum hydride produced organoaluminum imines having the formula RCH.dbd.NAl(i-C.sub.4 H.sub.9).sub.2, which were not isolated. L. I. Zakharkin et at., in both Bull. Acad. Sci. USSR, Engl. Trans., 1959, 523-524, and Proc. Acad. Sci. USSR, Engl. Trans., 1957, 112, 879-881. A gas consisting mainly of isobutene and polymers having the repeating unit --Al-N(R)].sub.n were produced upon heating the organoaluminum imine to 220.degree.-240.degree. C. and then subsequently raising the temperature to 280.degree. C. and maintaining this temperature for three hours. Products derived from hexane nitrile and anisonitrile were similarly prepared and heated at 260.degree.-300.degree. C. During the formation of the polymer, aluminum alkyl groups of the organoaluminum imine are eliminated as isobutene and aluminum-nitrogen bonds are formed.